Multifocal ophthalmic lenses are now well known. They are commonly used for correcting presbyopia, while allowing the wearer of glasses to observe objects over a wide range of distances without having to remove the glasses in order to view distant objects. Manufacturers of multifocal ophthalmic lenses usually manufacture a family of semi-finished lenses, i.e., initially, only that surface of the lenses in the family which includes the three above-mentioned vision zones is machined, while the other surface of the lenses in the family is machined subsequently to a spherical or toroidal shape with curvature appropriate for each wearer of glasses, depending on an ophthalmological prescription. In a given family of lenses, power addition varies gradually from one lens to another in the family between a value of minimum addition and a value of maximum addition. The minimum and maximum addition values are usually respectively 0.5 diopters and 3.5 diopters, and the addition varies in steps of 0.25 diopters from one lens to another in the family. Under such circumstances, a family of lenses comprises thirteen lenses.
Of the multifocal ophthalmic lenses that are commercially available, there are two main families of lenses. In the first family of lenses, the progression length, i.e. the distance between the first and third above-mentioned points of the progression main meridian curve is constant and the gradient of the optical power varies from one lens to another in said first family (French patent No. 2 058 499 and its two certificates of addition Nos. 2 079 663 and 2 193 989).
In the second family of lenses, the optical power gradient along the progression main meridian curve is constant and identical for all the lenses of said second family, regardless of their power addition (Japanese patent No. 54-85743).
It is well known, that regardless of the family to which they belong, multifocal ophthalmic lenses inevitably present optical aberrations (astigmatism, distortion, field curvature, etc.) that reduce comfort of vision, both in static vision and in dynamic vision. In addition, when the presbyopia of a person increases, requiring the use of lenses having greater power addition, changing to lenses having greater power addition usually requires a physiological adaptation effort on the part of the wearer of the glasses. The adaptation time may be one to several days depending on the subject.
In the past, the efforts of multifocal ophthalmic lens manufacturers have been directed mainly on improving comfort of vision.
Over the last few years, the Applicant has proposed a third family of ophthalmic lenses tending to resolve the problem of reducing physiological adaptation effort and adaptation time when changing from a pair of glasses having a first value of power addition to a pair of glasses having a second, greater value of addition (French patent No. 2 617 989).
It is now desired to satisfy even better the vision needs of people suffering from presbyopia by taking account, in particular, of their posture and of their habits, and also the shortening of distance to the work surface (reduction in the near vision distance) that is observed with increasing age of the subject.
An object of the present invention is thus to provide a multifocal ophthalmic lens, or more precisely a family of lenses, that takes account of the preferred up and down position of the eyes in the ocular orbit, which position itself depends on the viewing distance and on the inclination of the head in the sagittal plane (the vertical plane passing through the middle of the line segment joining the centers of rotation of the two eyes, and perpendicular thereto), and also taking account of the changes (reduction) in the near viewing distance with increasing age of the subject.